I have fuzzy balls attached to the under side of the leaf non my oak tree....
It sounds like oak galls. Oaks have lots of galls. Some are on leaves and others actually form on the twigs. Most are caused by tiny wasps that lay their eggs in the leaf or twig and cause the plant to form a ball of tissue around the developing larvae. There are different insects for each specific type of gall on each specific type of oak. The galls on woody twigs are unsightly in the winter when our northern oaks lose their leaves. Generally the galls cause little if any damage to the trees. Leaf galls are more a cosmetic problem than a threat to the tree. Chemical control with insecticides is difficult because the application of the proper material is based on the live cycle of the specific insect causing the gall
-Mark Longstroth SW Michigan Extension Fruit Educator Van Buren County MSU Extension Email - firstname.lastname@example.org